Imaging colorimeters are used to profile and calibrate the colorimetric performance of digital output devices, such as for example LCD (liquid crystal display) display panels, LED (light emitting diode) displays, and illuminated instrument clusters and keypads.
In a first prior art device shown in FIG. 1, such as for example the Cognex In-Sight 5705C from Natick, Mass., USA, a digital imaging sensor 100 is comprised of, for example, a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) sensor, and a rectangular array of red 120, green 130, and blue 140 color microfilters, which are bonded directly to sensor photosensitive cells (pixels or “pels”) 115. White light 110 incident upon the microfilters 120, 130, and 140 is filtered into red 125, green 135, and blue 145 light respectively. The color microfilters are arranged in a repeating “Bayer mosaic” pattern on the array's pels as represented by arrangements 150 (red), 160 (green), and 170 (blue). Advantages of this approach are low cost sensors and the lack of moving parts.
Unfortunately, this approach also has several disadvantages. First, the choice of spectral transmittance distributions of the red, green, and blue microfilters is severely limited by the availability of organic dyes that are compatible with the photoresist materials techniques required to fabricate the array. Second, the Bayer filter mosaic limits the color image resolution to 50% of the sensor resolution for green images, and 25% for red and blue images. Third, the interline CCD imaging sensors typically used for commercial imaging colorimeters have relatively small pels, which may limit the detector dynamic range and signal-to-noise ratio. Fourth, the pixels may not have identical spectral responsivity if the method for printing the filters is not highly reproducible.
In a second prior art embodiment shown in FIG. 2, such as for example the Prometric IC-PM from Radiant Vision Systems, Redmond, Wash., the imaging colorimeter 200 is comprised of an arrangement of imaging lenses 210, three or more color filters 220, 221, 222 mounted on a first mechanically rotatable disk 225, one or more neutral density filters 230, 231 mounted on a second mechanically rotatable disk 235, a mechanical or electro-optic shutter 240, and a digital image sensor 250. The lenses 210, selected color filter 221, selected neutral density filter 231, shutter 240 and image sensor 250 are aligned on a common optical axis 260.
In operation, a neutral density filter 231 (or none 232) is rotated into position, following which one of the color filters 221 is rotated into position prior to opening shutter 240 and capturing a digital image with image sensor 250. Each image is processed by an analog-to-digital converter and associated electronics module 270 and transmitted to a computer 280 for further processing or data storage.
An advantage of this approach is that individual red, green, and blue filters can be fabricated such that the combinations of their spectral transmittance distributions and the spectral responsivity distribution of the imaging sensor pels closely match the CIE color matching functions. A second advantage is that filters with different spectral transmittance distributions, including but not limited to narrowband, infrared, ultraviolet, and polarization filters, may be utilized for multiband spectral imaging applications. A third advantage is that the filtering method may provide a more uniform spectral responsivity than printed Bayer filters.
Unfortunately, this approach also has disadvantages. First, the need to physically rotate the color filter wheel necessarily limits the device throughput. The Prometric IC-PM colorimeters, for example, may have long measurement times due to resolution-dependent image sensor read-out time and filter wheel rotation speed. This can be a disadvantage for production line testing, as it may represent a bottleneck in the production flow.
A second disadvantage is that the rotating filter wheel introduces moving parts that are subject to vibration, wear, and possible failure, while a third disadvantage is that the spectral range is limited to that of the spectral responsivity distribution of the imaging sensor, for example as shown in FIG. 3. This eliminates the possibility of, for example, multiband spectral imaging involving both ultraviolet and infrared radiation bands.